Optical receivers facilitate data transfer by detecting optical signals transmitted over optical fiber cables in optical communications networks. In such networks, an optical transmitter modulates optical signals at high frequencies to send them over a fiber using one or more optical wavelengths. To receive the transmitted signals, optical receivers typically use avalanche photodiodes (“APDs”) to detect the optical signals. As is known, an APD is a photodiode that has an internally-generated multiplication layer where multiplication of initial photocurrent occurs. The operation of the APD, however, is susceptible to noise and variations in temperature. As such, measuring the photocurrent and adjusting the bias voltage are both necessary to ensure proper APD operation. But conventional structures and techniques for facilitating photocurrent measurements and bias voltage adjustments have several drawbacks.
FIG. 1 is a conventional configuration network 100 for long distance communications. Generally, an optical network includes many transmitters and receivers. For illustration purposes, FIG. 1 shows a multiplexer (“MUX”) 106 representing one or more transmitters. Multiplexer operates to combine “n” optical signals 104 of different wavelengths for transmission over an optical fiber 108 as modulated optical signals. A demultiplexed (“DEMUX”) 110 representing one or more receivers operating to separate the previously combined signals into optical signals 112. To monitor the operation and quality of optical network, an external power monitor 120 is used to measure the input optical power for any given wavelength of optical signals 112. Typically, external power monitor 120 is used to measure optical power external to a housing that contains an optical receiver device. Drawbacks to this approach to monitoring power are plain. Additional external equipment and resources (e.g., labor, time, etc.) are required to construct this configuration 100 to include external power monitor 120. While configuration 100 can be used to accurately measure power, it is usually used only once, such as during manufacturing of the optical receiver. As such, in-line APD gain adjustments are generally not feasible during normal operations of the APD, especially when it is being used in a communications network. Another drawback to configuration 100 is that external power monitor 120 introduces an additional component causing signal losses in the optical path, thereby degrading optical power monitoring.
FIG. 2A depicts an approach to optical signal power monitoring in which input optical power is monitored internally. In this approach, input optical power, which is derivable from measuring photocurrent, is performed at the output of trans-impedance amplifier (“TIA”). As shown, structure 200 includes an APD 202 to detect optical signals 206, and a trans-impedance amplifier 204 for measuring photocurrent in terms of voltage 208. But there are several drawbacks to this approach to monitoring input optical power. One drawback is that precise signal splitting is technically difficult without disturbing the high-frequency nature of main output signal 208. In particular, elements 231 and 233, which include resistors, R, and capacitors, C, are used to split a portion of main output signal 208 to form a signal portion 209. In this approach, signal portion 209 is used to measure the optical power. But with main output signal 208 commonly being at high frequencies and data transfer rates, such as at 10 Gbps or higher, it is difficult to effectively split the signal for accurate power monitoring. FIG. 2B shows that another drawback to structure 200 in monitoring power is that it has a relatively high-degree of non-linearity, which significantly reduces the total range over which to monitor optical power.
FIG. 2B is a graph 250 depicting the relationship between photocurrent and the output of the trans-impedance amplifier 204 (FIG. 2A). Range 254 is a non-linear range; minute linear changes in photocurrent lead to drastic changes in the trans-impedance amplifier output. It is in this range that trans-impedance amplifier 204 (FIG. 2A) is generally highly saturated. The non-linearity of range 254 reduces the effective range of monitoring power at high input optical power values (e.g., −18 dBm to −3 dBm). As it is desirable to set an alarm limit (e.g., at −3 dBm) to protect an APD from, for example, over-current events, range 254 renders trans-impedance amplifier 204 (FIG. 2A) unusable for detecting unsafe operating conditions.
FIG. 3 depicts another power monitoring approach in which power is monitored internally, and more specifically, prior to the current reaching the trans-impedance amplifier. To internally monitor optical power incident to an APD 320, configuration 300 includes a bias voltage-setting circuit 301, an input stage amplifier 302, a current mirror 304, and a logarithmic amplifier (“log amp”) 310. Bias voltage-setting circuit 301 operates to generate a bias voltage at input 311 of input stage amplifier 302, which in turn functions to generate output 313 to bias the input APD voltage at node 303. Unlike the approach described in FIGS. 2A and 2B, trans-impedance amplifier (“TIA”) 312 is used only to amplify electrical signals representing the data to be communicated; it is not used for power monitoring purposes. But note that current mirror 304 is included in the current measurement path to logarithmic amplifier (“log amp”) 310 for power monitoring purposes. In some alternate structures for configuration 300, current mirror 304 includes a high-voltage transistor (“HV Trans”) 306.
In view of the foregoing, it would be desirable to provide an apparatus and a method that minimizes the above-mentioned drawbacks, thereby facilitating power monitoring using an extended range for adjusting gain and controlling unsafe conditions, among other things, especially while operating at different temperatures.